Researcher Finds Solar Trend That Can Warm ClimateEnds debate over whether sun can play a role in climate change

The sun on
6/28/2000 during the current solar cycle 23 activity maximum.
This photo was taken in a far ultraviolet wavelength (the He
II line at 30.4 nanometers) and emphasizes the enhanced radiative
activity of magnetically active regions. Image from the MDI
experiments archive.

New data indicate that the sun may contribute
to global climate change, according to a new study by Richard Willson,
a Columbia-affiliated researcher.

Since the late 1970s, the amount of solar radiation
the sun emits during times of quiet sunspot activity has increased
by nearly .05 percent per decade, according to the study. “This
trend is important because, if sustained over many decades, it
could cause significant climate change,” said Willson, a
researcher affiliated with NASA Goddard Institute for Space Studies
and the Earth Institute at Columbia University, and lead author
of the study recently published in Geophysical Research Letters.

“Historical records of solar activity
indicate that solar radiation has been increasing since the late
19th century,” says Willson. “If a trend comparable
the one found in this study persisted during the 20th century it
would have provided a significant component of the global warming
that the Intergovernmental Panel on Climate Change report claims
to have occurred over the last 100 years.”

Willson found errors in previous satellite
data that had obscured the trend. The new analysis, Willson says,
should put an end to a debate in the field over whether solar irradiance
variability can play a significant role in climate change.

The solar cycle occurs approximately every
11 years when the sun undergoes a period of increased magnetic
and sunspot activity called the "solar maximum," followed
by a quiet period called the "solar minimum." A trend
in the average solar radiation level over many solar magnetic cycles
would contribute to climate change in a major way. Satellite observations
of total solar irradiance have now obtained a long enough record
(over 24 years) to begin looking for this effect.

The
computer graphic animation shows the ACRIMSAT/ACRIM3
satellite/experiment deploying its solar panels and
spinning-up following launch. The solar panels provide
power to operate the dedicated ACRIMSAT satellite and
its ACRIM3 experiment. The cylindrical ACRIM3 instrument
is seen, with its three sensor apertures, in the center
of the satellite. The satellite is precisely pointed
at the sun using the spin stabilization shown in the
animation. The ACRIM3 experiment began monitoring the
total solar irradiance in April 2000 and is designed
for a minimum 5 year mission. Its 700 km Earth orbit
will provide measurement opportunities lasting much
longer. The actual lifetime of the experiment will
depend on the reliability of its electronic systems.

Total Solar Irradiance (TSI) is the radiant
energy received by the Earth from the sun over all wavelengths
outside the Earth's atmosphere. Its interaction with the Earth’s
atmosphere, oceans and land masses is the biggest factor determining
the Earth’s climate. To put it into perspective, decreases
in TSI of 0.2 percent occur during the week-long passage of large
sunspot groups across our side of the sun. These changes are relatively
insignificant compared to the sun’s total output of energy,
but are equivalent to all the energy that mankind uses in a year.
According to Willson, small variations like the one found in this
study, if sustained over many decades, could have significant climate
effects.

In order to investigate the possibility of
a solar trend, Willson needed to put together a long-term dataset
of the Sun’s total output. Six overlapping satellite experiments
have monitored TSI since late 1978.The first record came from the
National Oceanic and Atmospheric Administration’s (NOAA)
Nimbus7 Earth Radiation Budget (ERB) experiment (1978-1993). Other
records came from NASA’s Active Cavity Radiometer Irradiance
Monitors: ACRIM1 on the Solar Maximum Mission (1980-1989), ACRIM2
on the Upper Atmosphere Research Satellite (1991-2001) and ACRIM3
on the ACRIMSAT satellite (2000 to present). Also, NASA launched
its own Earth Radiation Budget Experiment on its Earth Radiation
Budget Satellite (ERBS) in 1984. And, the European Space Agency’s
(ESA) SOHO/VIRGO experiment also provided an independent data set
during 1996-1998.

In this study, Willson, who is also Principal
Investigator of the ACRIM experiments, compiled a TSI record of
over 24 years by carefully piecing together the overlapping records.
In order to construct a long-term dataset, Willson needed to bridge
a two-year gap (1989-1991) between ACRIM1 and ACRIM2. Both the
Nimbus7/ERB and ERBS measurements overlapped the ACRIM ‘gap.’ Using
Nimbus7/ERB results produced a 0.05 percent per decade upward trend
between solar minima, while ERBS results produced no trend. Until
this study, the cause of this difference, and hence the validity
of the TSI trend, was uncertain. Now, Willson has identified specific
errors in the ERBS data responsible for the difference. The accurate
long-term dataset therefore shows a significant positive trend
(.05 percent per decade) in TSI between the solar minima of solar
cycles 21 to 23 (1978 to present).

The ACRIMSAT/ACRIM3 experiment began in 2000
and will carry out long-term solar observations for at least five
more years. The instrumentation for the ACRIMSAT/ACRIM3 experiment
was the latest in a series of ACRIM’s developed for satellite
experiments by Willson and the Jet Propulsion Laboratory (JPL)
of the California Institute of Technology. JPL operates the ACRIMSAT/ACRIM3
experiment for Willson using their tracking station at the Table
Mountain Observatory in California. One of the missions of NASA’s
Earth Science Enterprise, which funded this research, is to study
the primary causes of climate variability, including trends in
solar radiation that may be a factor in global climate change.
For more information about ACRIM, please go to: http://www.acrim.com

The Earth Institute at Columbia University is among the
world’s leading academic centers for the integrated study of Earth,
its environment, and society. The Earth Institute builds upon excellence
in the core disciplines—earth sciences, biological sciences, engineering
sciences, social sciences and health sciences—and stresses cross-disciplinary
approaches to complex problems. Through its research, training and global
partnerships, it mobilizes science and technology to advance sustainable
development, while placing special emphasis on the needs of the world’s
poor.